1. Field of the Invention
The present invention relates to optical temperature measurement. In particular, the present invention relates to a method and apparatus for measuring temperature by analyzing temperature-dependent changes in light transmission through temperature-sensitive materials.
2. Discussion of Background
Measurement and control of temperature are important in many industrial, medical, household, and research applications. Temperature monitoring and control systems are found in nuclear reactor vessels and coolant systems, treatment facilities for administration of chemotherapy or radiotherapy, conventional and microwave ovens, and various industrial processes. Temperature is often monitored at underground nuclear waste-disposal sites, chemical dumping sites, geothermal wells, and the like. Temperature measurements in hazardous or inaccessible locations must be carried out remotely. Remote measurements are indicated in regions having high radiation levels, high intensity electric or magnetic fields, high pressures and temperatures, toxic gases, corrosive materials or the like.
Temperature measurements can be made with well-known devices such as thermocouples, thermistors, resistance thermometers and the like. These devices contain electrically-conducting components that generate temperature-dependent electrical signals. The signals are amplified and converted into temperature readings or used in control functions. Such devices often give erroneous readings due to electrical interference problems. The devices are subject to field perturbation effects when used in the presence of electromagnetic fields such as those produced by electric motors, generators, power cables and the like. Some of these devices present safety hazards to personnel because of possible high voltages induced or conducted from high voltage sources. In addition, some devices are subject to degradation in severe operating environments.
Many of these problems can be overcome by using remote, in situ optical sensors coupled to a detector by optical fibers. Optical sensors contain essentially no metallic or electrically-conducting components. Optical fibers are durable, corrosion-resistant, heat-resistant, and impervious to electrical or magnetic interference. The fibers allow remote monitoring of sensors in inaccessible and/or hazardous locations. In addition, signals can be transmitted over optical fibers with low attenuation and without prior conversion or conditioning. The information-carrying capacity of optical fibers is greater and less subject to interference than that of electrical cables. The signals can be multiplexed so that a single light source and a single light detector can be used for measuring the outputs of many remote sensors.
Many types of optical temperature sensor are available. Several devices use optical time-domain reflectometry (OTDR) techniques to measure temperature (Grego, U.S. Pat. No. 4,830,513; Hartog et al, U.S. Pat. Nos. 4,823,166; 4,767,219; Bibby, GB Patent No. 2,170,594; Dakin, GB Patent No. 2,140,554). Other sensors measure temperature-dependent changes in the absorption spectra of materials such as GaAs, CdTe, and CdS (Christensen, U.S. Pat. No. 4,790,669; Ida, U.S. Pat. No. 4,669,872; Salour, et al., U.S. Pat. No. 4,703,175), and temperature-dependent absorption in neodymium-doped glass (Kleinerman, U.S. Pat. No. 4,708,494; Dakin, U.S. Pat. No. 4,673,299; Snitzer et al., U.S. Pat. No. 4,302,970).
The fluorescence intensities of glasses or ceramics doped with neodymium (Angel, et al., U.S. Pat. No. 4,729,668; Bijlenga et al., U.S. Pat. No. 4,592,664) and lithium neodymium phosphate (Hirano et al., U.S. Pat. No. 4,679,157) are used for temperature measurements. Sensors based on temperature-dependent fluorescent or phosphorescent properties of materials include a device for measuring the intensity ratio of two distinct and optically-isolatable fluorescent emission lines of selected rare earth-doped compounds (Wickersheim, U.S. Pat. Nos. 4,448,547; 4,560,286; 4,215,275; 4,075,493).
All of these techniques are subject to inaccuracy caused by poor resolution, drift, variable optical losses in the transmitting fibers, difficulties in measuring light transmission through an optical fiber undergoing changes in temperature, and limited temperature ranges. The measurements may be affected by thermal expansion/contraction of the probe materials, so each probe must be individually calibrated before use. There is a need for an inexpensive optical temperature sensor method and apparatus that provides accurate, reproducible data over a broad temperature range without the need for extensive calibration procedures.